The present invention relates to the field of photography and, more specifically, to automatic exposure control systems and methods incorporating a microcomputer for developing a film exposure strategy or program based on certain pre-exposure scene information inputs and for, thereafter, controlling operation of selected system components during the film exposure phase to carry out the developed strategy.
"Frame the scene in the viewfinder and then press the cycle start button!" With today's fully automatic cameras, that is about all the instruction anyone needs to take highly acceptable photographs on a consistent basis.
The extent to which cameras aimed at the mass amateur market have been automated is typified by the self-developing SX-70 and Pronto! Autofocus Land Cameras marketed by Polaroid Corporation, Cambridge, Mass. In response to actuation of the automatic camera cycle, a sonar ranging system measures camera-to-scene distance and sets the objective lens at the correct focus position, the film unit is automatically exposed in accordance with the scene brightness level measured by a photocell circuit, and then the exposed film unit is advanced automatically between a pair of pressure applying rollers to initiate a well-known development and diffusion transfer process.
These cameras also are configured for automatic flash mode operation employing flashbars (an array of flash-bulbs) and/or strobe units. Not only do such cameras operate in a conventional flash mode when ambient light is very low so that exposure illumination is provided primarily by the artificial light source, but they also operate in an automatic fill flash mode at higher ambient light levels. In this mode, artificial illumination is mixed proportionally with ambient light for the purpose of eliminating or softening shadows, or for adding additional illumination to dark areas of the scene to bring out the details thereof.
In the above-noted cameras, transmission of image forming light to the film plane is controlled by a dynamic aperture scanning shutter which employs an integrated blade mechanism having a pair of counter-reciprocating blades for defining both effective aperture and exposure interval.
The blades are displaceable between a closed light blocking position and a fully open position where the blades define a maximum obtainable aperture. As the blades open, the aperture progressively increases in size (area) until a peak aperture, determined by appropriate exposure parameters, is reached. Thereafter the direction of blade displacement is reversed and the effective aperture progressively decreases in size until the shutter closes to terminate the exposure interval.
Operation of the blade mechanism may be characterized by an aperture size versus time (exposure interval) trajectory curve that provides both quantitative and qualitative information about the resultant exposure.
When the intensity of the image forming light is known, the area bounded by the curve is indicative of the exposure value or total amount of light reaching the film plane.
The general shape of the trajectory curve determines the nature of the exposure in terms of motion stopping ability, depth of field and other parameters. For example, the blade mechanism may be driven rapidly to quickly reach a relatively large peak aperture and then close in the same manner. The exposure interval is relatively short providing good motion stopping ability. However, depth of field will suffer somewhat because of the large peak aperture. For the same exposure value, the blade mechanism may be driven more slowly and attain a smaller peak aperture to improve depth of field, but the exposure interval must be increased accordingly and motion stopping ability will be diminished. For general picture taking situations, however, the trajectory curve shape should fall somewhere between these two extremes and provide an optimized balance between motion stopping ability and depth of field.
In recent years, there have been dramatic improvements in the image quality of photographs made with fully automatic cameras. Some of these improvements result from better shutter mechanisms, increased precision in measuring scene brightness levels over a wide range of conditions and distances, and more accurate control over the entire exposure system. Other improvements have been obtained by adding new subsystems, such as automatic ranging and focusing, and proportional fill flash.
For the most part, advances in image quality have been made by automating well-known exposure methods that previously required the operator to have extensive understanding of the photographic process.
Now that this approach has been extensively exploited, there are many that believe that future improvements in image quality will be derived mainly from the development of new exposure methods and/or significant modifications of contemporary techniques.
In the dynamic aperture shutter field there has been some work done exploring the effect of trajectory curve shape on depth of field and motion stopping ability parameters and suggesting optimized balanced trajectories that are generally triangular in shape but include opening and closing portions (sides) that are somewhat concave. For example, see "Photographic Shutters: Better Pictures With A Recondisderation Of Shutter Efficiency" by William T. Plummer, Applied Optics, vol. 16, page 1914, July 1977.
There are many questions yet left to be answered. For example, how should the trajectory be shaped for optimized flash and fill flash photography? Can the trajectory curve be shaped to exploit the advantages of a given imaging optical system while minimizing its drawbacks? There are still legitimate debates about the relative merits of follow focus, strobe quench and hybrid flash systems. Automatic fill flash is relatively new. What is the optimum ratio of ambient to strobe light? when the ambient brightness level is sufficient to provide a predetermined level of natural light contribution for the exposure, should an attempt be made to keep this ratio constant or should it be varied as a function of scene brightness level and/or distance?
What is needed to keep pace with advances in exposure techniques is a truly versatile exposure control system. That is, a system that can automatically provide a great variety of different trajectory shapes, in response to various inputs, that will define the nature of the exposure and also easily manipulate the complex interactive variables of exposure decision making to control ambient and flash modes of operation.
Commonly assigned copending application U.S. Ser. No. 216,831, filed on Dec. 16, 1980, discloses an exposure control system that is configured for ambient light operation and exhibits the required versatility for exploring new exposure techniques in that its shutter operation is controlled by a microcomputer which is programmed to provide a wide variety of different trajectory curve shapes in response to automatic inputs indicative of scene conditions and/or manual inputs relating to operator preferences for curves that emphasize either depth of field or motion stopping ability.
The shutter blade mechanism is driven by a stepper motor which responds to a selected program of trajectory signals for displacing the blades in a manner to produce a corresponding trajectory defined by the signal program. In response to various pre-exposure inputs, including scene ambient brightness level, the computer utilizes a preprogrammed trajectory data base to develop an appropriate program of trajectory signals which it then feeds to the stepper motor.
The present invention is directed to an expansion of this system's capabilities by adding a strobe flash subsystem that is highly versatile and takes a new approach to flash mode operation. Not only is the flash subsystem compatible with the stepper motor driven shutter, but it may be added at a relatively low cost because its operation is programmed and controlled by a moderately expanded version of the same microcomputer that operates the exposure control system in the ambient light exposure mode.
As noted earlier, proportional fill flash systems are well known in the prior art. When ambient light levels are relatively low, most of the exposure illumination is provided by the strobe unit. However, when the ambient brightness level increases to a predetermined level so that natural light may provide a fixed percentage (e.g., 75%) of the optimum exposure value, the strobe output level is automatically adjusted, as a function of subject distance and ambient scene brightness level, to provide the remaining percentage of light needed to reach full exposure value.
There are numerous approaches to providing proportional fill flash. For example, some systems operate in a follow focus mode whereby a fixed output strobe is fired at an aperture that is correlated to scene distance. Other systems utilize strobe quenching techniques and fire the strobe at the peak aperture, set for the ambient contribution in accordance with scene brightness level or other strobe firing aperture, and vary the strobe's output in proportion to scene distance. Still other systems combine these two techniques by using follow focus for scene distances up to 9 to 10 feet and then shifting to a quench method for scene distances beyond this range. For representative examples of these systems, reference may be had to commonly-assigned copending application, U.S. Ser. No. 135,521, filed on Mar. 31, 1980 and entitled "Exposure And Flash Fire Control System" and the additional references set forth therein.
Also, a number of prior art references are relevant for showing automatic flash firing systems wherein digital computation techniques are used to determine the appropriate flash firing parameters based on scene range and ambient brightness level inputs. For representative examples, see U.S. Pat. Nos. RE-29,599; 3,928,859; 4,005,444; 3,836,920; 4,187,019; 4,024,549; 3,710,701; and 3,742,828. U.S. Pat. No. 4,035,814 discloses an exposure control system that utilizes a digital counter memory subsystem for determining flash parameters in response to scene brightness and distance inputs. U.S. Pat. No. 4,065,776 is relevant for disclosing a flash fire system configured for use with a stepper motor driven shutter. U.S. Pat. Nos. 3,738,242 and 4,236,794 are relevant for showing photographic systems wherein at least a part of the photographic processes is controlled by digital computer that includes a preprogrammed data base or matrix table.
A common drawback of these prior art exposure control systems is that they tend to be unduly constrained and limited in their operation, thereby lacking the versatility required to approach optimizing the different exposure modes and options. That is, some systems only can provide one general trajectory shape and this is usually optimized for ambient mode operation. Therefore, there is a prior restraint of having to make the flash mode operation conform to the ambient trajectory shape. Other systems may change the trajectory for better flash mode operation, but suffer in the fill flash mode because the ambient contribution is made with a trajectory shape that may be optimized for strobe contribution at low ambient light levels.
Therefore, it is an object of the present invention to provide a highly versatile automatic exposure control system for ambient light and strobe flash photography.
It is another object to provide such an exposure control system that is easily to adaptable to new exposure techniques.
It is yet another object to provide such a system that is capable of extending the useful range of a given strobe unit by employing a unique method of selecting flash mode parameters.
Still another object of this invention is to provide such a system that is programmable.
Another object is to provide an automatic exposure control system that is controlled by a microcomputer which utilizes preprogrammed trajectory curve and strobe operation data bases in the automatic strobe mode exposure decision making process.
It is also an object of this invention to provide photographic apparatus, including a camera, incorporating such an automatic exposure control system.
Other objects of the invention will, in part, be obvious and will, in part, appear hereinafter.